A liquid crystal display (LCD), used in television, personal computer, mobile phone and the like, has a liquid crystal panel that includes a liquid crystal cell and polarizing plates placed on both sides of the liquid crystal cell so that it enables display by controlling an amount of transmission of light emitted from a light source. As LCDs have been rapidly developed, their functions and applications tend to be diversified, and they are required to withstand more harsh environments during use. For example, LCDs for mobile applications such as cellular phones and for vehicle applications such as car navigation systems are required to have durability even to more harsh environments during use. In addition, large-sized LCDs such as LCD televisions are required to have durability to higher temperature, because as their size and brightness increase, an amount of heat from the light source increases, so that the LCDs themselves tend to have high temperature. Thus, polarizing plates for use in LCDs are also required to have less changeable properties under harsh environments such as high temperature and high humidity, namely, to have high durability.
A polarizing plate generally has a structure in which a polarizer is provided between two protective films. Triacetylcellulose (TAC) is widely used for protective films. In view of durability as mentioned above, it is proposed to use, as a polarizer-protecting film, a film having a high level of mechanical properties, chemical resistance, and water-barrier properties, such as a polyethylene terephthalate (PET) film or a polyethylene naphthalate (PEN) film (see for example Patent Document 1).
A polyester film such as a PET or PEN film is highly stretched and crystallized and therefore has a high level of mechanical properties and other properties as mentioned above. However, the polyester film has high birefringence in the in-plane direction and the thickness direction, because its intrinsic birefringence is high and is highly stretched. Therefore, when a film made of a high-birefringence material, such as a polyester film is placed as a polarizer-protective-film between a polarizer and a liquid crystal cell, polarization is distorted between the polarizer and the liquid crystal cell due to an effect of the birefringence, so that visibility may tend to be significantly reduced. From this point of view, such a polyester film is generally used as a protective film for the principal surface of the polarizer on the side not facing the liquid crystal cell.
Such a film provided to protect the principal surface of the polarizer on the side not facing the liquid crystal cell does not change the polarization between the polarizer and the liquid crystal cell. Therefore, the effect of the birefringence on the display properties of the liquid crystal display is small, and the film is not necessarily required to have optical isotropy or birefringence uniformity. However, there is a problem in which when a polyester film is used as a protective film for the principal surface of the polarizer on the side not facing the liquid crystal cell in a polarizing plate placed on the light source side of a liquid crystal cell, iridescent unevenness (rainbow like unevenness) occurs to degrade visibility. Particularly, as the brightness and color purity of LCDs have increased in recent years, such iridescent unevenness becomes more likely to be visible, which becomes a hindrance to the use of a polyester film as a polarizer-protective-film.
From this point of view, it is proposed that a light scattering layer should be provided on the surface of a polarizer-protective-film, especially in a polarizing plate placed on the viewer side of the liquid crystal cell, so that mixing of optical paths, namely, mixing of colors can be made to make iridescent unevenness less visible. For example, Patent Document 2 discloses that when a light scattering layer is provided on the surface of a polyester film with an in-plane retardation of 500 nm or more to form a polarizer-protective-film, such iridescent unevenness can be reduced. Such a method can make iridescent unevenness less visible but does not prevent iridescent unevenness itself, so that sufficient visibility cannot be obtained in some cases. As the brightness and color purity of LCDs further increase, the occurrence of iridescent unevenness becomes more significant. It is therefore considered that it will be difficult to ensure sufficient visibility by such a color mixing method for canceling iridescent unevenness.